1.3Geological TimePeriods and Changes in Organisms

The first livingorganisms to appear on the earth are thought to have been anaerobicunicellular organisms, who usedmarineorganicsubstances without usingoxygen (Fig. 1-2). Let us nowlook at the changes that occurred in organisms over time according to the divisions of geological timeperiods.
The period from the formation of the earth until 560 millionyears ago is called the Precambrianage, and the first life formappeared during this time. Later,photosynthetic bacteria and cyanobacteriaappeared in the ocean. These bacteria were able to synthesizeorganicsubstancesusingcarbon dioxide,thuscausingoxygen to graduallyincrease in the atmosphere.Organisms became multicellular, and eukaryotesemerged(seeSection 4 of Chapter 1). Radiolarians (protozoa), sponges, and green algaeemerged at the end of the Precambrianage.

Concurrently, the increasedoxygen was changed to ozone by the ultraviolet rays in the stratosphere 10–50 km above the earth. This ozoneformed a layer that blocked the harmfulultraviolet rays,preventing them from reaching the earth's surface. This conditionenabledlivingorganisms to advance from sea to land. In the Paleozoic era about 400 millionyears ago, the first organisms to advance to land were bryophytes.
During the Paleozoic era,fishes and amphibiansappeared and flourished in water, and ferns flourished on land. In the Mesozoic era,reptiles such as dinosaurs flourished, and gymnospermous plants such as conifersdominated the ecosystem. The Cenozoic erabegan when largereptilesgradually became extinctafter the earth was struck by a meteorite, ushering in the era of angiosperms and mammals,includinghumans.